In industrial measurements technology, electronic devices, especially measuring devices, are regularly used in locations where a danger of explosion exists. Examples include flour silos, gas, i.e. fueling, stations, and chemical plants, where explosive gases can exist.
Commonly used measuring devices include e.g. flow, fill-level and pressure measuring devices.
Special safety requirements are placed on devices usable in explosion-endangered areas. These requirements have the goal of avoiding spark formation, which could, in the right circumstances, lead to an explosion, or of preventing that a spark occurring in a sealed chamber has any effects on the environment of the chamber. Such a goal is achievable in different ways, which are referred to in corresponding European standards as ignition protection types.
Thus e.g. according to European Standard EN 50 020 of the year 1994, explosion protection is given, when devices are built according to the protection class defined therein named ‘Intrinsic Safety’ (Ex-i). According to this protection class, the values for the electrical variables, current, voltage and power, in a device have to lie at all times each below a predetermined limit value. The three limit values are so selected that, in the case of malfunction, e.g. due to a short circuit, the maximum occurring release of energy is not sufficient to produce an ignition spark. The variables are maintained below their respective predetermined limit values as follows: Electrical current e.g. by resistors, voltage e.g. by Zener diodes, and power by appropriate combinations of current and voltage limiting elements.
Another protection class is described in the European Standard EN 50 018 of the year 1994 under the title ‘Pressure-Resistant Encapsulation’ (Ex-d). Devices built according to this protection class have a pressure-resistant housing, by which is assured that an explosion occurring in the interior of the housing can not be transmitted outside. Pressure-resistant housings are, in order that they have a sufficient mechanical strength, thick walled and, thus, heavy and expensive.
USA, Canada, Japan and other countries have standards comparable with these European standards.
The requirements of explosion protection hold true, naturally, not only for the actual measuring device, but also for display- and/or operating-elements used in explosion-endangered areas. Display elements serve e.g. for displaying measured values on-site. Operating elements are used e.g. for adjusting device settings or inputting operating parameters on-site.
Display- and/or operating-elements are, for reasons of explosion protection, today frequently located in inner spaces of the housing of the measuring device or in separate housing parts, which are sealed e.g. by means of a pressure-resistant lid. Such lids include, as a rule, a viewing window, for enabling the display to be observed. It is possible to equip the operating element with optical buttons, or keys, which can be operated through the viewing window. More economical, mechanical buttons cannot be operated through the lid. The lid represents an additional component, with which increased production costs are associated.